1. Field of the Invention
The present invention relates to processes for the partial oxidation in a membrane wall gasification reactor of heavy bottoms that can also contain waste materials recovered from a solvent deasphalting unit operation to produce a high value synthesis gas.
2. Description of Related Art
Solvent deasphalting is a process employed in oil refineries to extract valuable components from residual oil. The extracted components can be further processed in refineries where they are cracked and converted into lighter fractions, such as gasoline and diesel. Suitable residual oil feedstocks which can be used in solvent deasphalting processes include, for example, atmospheric distillation bottoms, vacuum bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands. Solvent deasphalting processes are well known and described, for instance, in U.S. Pat. Nos. 3,968,023, 4,017,383, and 4,125,458, the disclosures of which are incorporated herein by reference.
In a typical solvent deasphalting process, a light hydrocarbon solvent, which can be a combination of one or more paraffinic compounds, is admixed with a residual oil feed to flocculate and separate the solids from the oil mixture. Common solvents and their mixtures used in the deasphalting process include normal and/or iso-paraffins with carbon numbers ranging from 1 to 7, preferably from 3 to 7, including most preferably, propanes, normal and/or iso butanes, hexanes, and heptanes. Under elevated temperatures and pressures, generally below the critical temperature of the solvent, in an asphaltene separator, the mixture is separated into two liquid streams, including (1) a substantially asphaltenes-free stream of deasphalted oil that includes resins, and (2) a mixture of asphaltenes and solvent that includes some dissolved deasphalted oil.
The substantially asphaltenes-free mixture of deasphalted oil and solvent is normally passed to a solvent recovery system. The solvent recovery system of a solvent deasphalting unit extracts a fraction of the solvent from the solvent-rich deasphalted oil by boiling off the solvent, commonly using steam or hot oil from heaters. The solvent is recycled and sent back for use in the solvent deasphalting unit.
In some processes, the deasphalted oil fraction is also separated into a resins fraction and a resins-free fraction. “Resins” as used herein means materials that have been separated and obtained from a solvent deasphalting unit. Resins are denser and heavier than deasphalted oil, e.g., maltenes, but lighter than asphaltenes. The resins product usually comprises aromatic hydrocarbons with highly aliphatic-substituted side chains, and can also include metals, such as nickel and vanadium.
An enhanced solvent deasphalting process is disclosed in U.S. Pat. No. 7,566,394 in which a hydrocarbon oil feedstock containing asphaltenes is introduced into a mixing vessel with a paraffinic solvent and a solid adsorbent material. The solid adsorbent material can include attapulgus clay, alumina, silica activated carbon and zeolite catalyst materials, and combinations of those materials. The solid asphaltenes formed in the paraffinic solvent phase are mixed with the adsorbent material for a time sufficient to adsorb sulfur- and nitrogen-containing heavy polynuclear aromatic molecules present in the hydrocarbon oil feed on the adsorbent material. The solid phase comprising asphaltenes and adsorbent is separated from the oil/solvent mixture. The oil/solvent mixture is passed to a separation vessel to be separated into deasphalted oil and paraffinic solvent. The paraffinic solvent is recovered and recycled to the mixing vessel.
The asphalt/adsorbent material mixture is passed to a filtration vessel with an aromatic or polar solvent. The solvent can comprise benzene, toluene, xylenes, or tetrahydrofuran. The adsorbent materials are separated and cleaned in the filtration vessel. The cleaned solid adsorbent is recovered and recycled to the mixing vessel. The asphalt material is extracted from solid adsorbent materials and can be used directly as an asphalt component, or blended in an asphalt pool. The aromatic or polar solvent mixture is then passed to a fractionator to recover the solvent for recycling to the filtration vessel. Process reject materials including heavy polynuclear hydrocarbons with a high concentration of nitrogen and sulfur compounds are discharged from the fractionator.
During the solvent deasphalting process described above, the adsorbent material(s) must be reconditioned and/or removed after their adsorbent capacity falls below a desired efficacy, i.e., they are deemed to be spent. The spent adsorbent includes constituents such as heavy polynuclear aromatic molecules, sulfur, nitrogen and/or metals. Disposal of the spent adsorbent as waste materials incurs substantial expense and entails environmental considerations.
In addition, when adsorbent materials are reconditioned, for example, by solvent desorption, heat desorption or pyrolysis at high temperatures, the process reject materials removed from the adsorbent materials must also be disposed of appropriately. These process reject materials can include heavy hydrocarbon molecules containing sulfur, nitrogen and/or heavy aromatic molecules, and metals such as nickel and vanadium.
A process is described in U.S. Patent Publication Number 2009/0301931 for the disposal of refinery process waste including spent catalytic and non-catalytic adsorbent materials and process reject materials remaining after their desorption from solid adsorbent material(s). As an example, solvent deasphalting bottoms recovered from a solvent deasphalting process such as that described in U.S. Pat. No. 7,566,394 are collected in an asphalt pool and used as paving materials or cutback asphalt. In particular, spent solid adsorbent material and asphalt are discharged from the filtration vessel to an asphalt pool. Process reject materials, which include bituminous materials, are discharged from the fractionator to the asphalt pool. A portion of the asphalt and adsorbent material mixture can also be discharged directly into the asphalt pool, for example, if the adsorbent materials are not to be recycled back to the mixing vessel of the enhanced solvent deasphalting unit. The disposal method described in U.S. Patent Publication Number 2009/0301931 is one option to dispose of the process reject material from solvent deasphalting; however, it may not be cost-effective due to the existing refinery infrastructure or the feedstock and/or product cost margins. Therefore, a need exists for an alternative and cost-effective solution for disposal of solvent deasphalting process bottoms while minimizing conventional waste handling demands.
Gasification is well known in the art and it is practiced worldwide with application to solid and heavy liquid fossil fuels, including refinery bottoms. The gasification process uses partial oxidation to convert carbonaceous materials, such as coal, petroleum, biofuel, or biomass with oxygen at high temperature, i.e., greater than 800° C., into synthesis gas, steam and electricity. The synthesis gas consisting of carbon monoxide and hydrogen can be burned directly in internal combustion engines, or used in the manufacture of various chemicals, such as methanol via known synthesis processes and synthetic fuels via the Fischer-Tropsch process.
The major benefits for a refinery using a heavy residue gasification process are that it can also provide a source of hydrogen for hydroprocessing to meet the demand for light products; it produces electricity and steam for refinery use or for export and sale; it can take advantage of efficient power generation technology as compared to conventional technologies that combust the heavy residue; and it produces lower pollutant emissions as compared to conventional technologies that combust heavy residues as a means of their disposal. Furthermore, the gasification process provides a local solution for the heavy residues where they are produced, thus avoiding transportation off-site or storage; it also provides the potential for disposal of other refinery waste streams, including hazardous materials; and a potential carbon management tool, i.e., a carbon dioxide capture option is provided if required by the local regulatory system.
Three principal types of gasifier technologies are moving bed, fluidized bed and entrained-flow systems. Each of the three types can be used with solid fuels, but only the entrained-flow reactor has been demonstrated to process liquid fuels. In an entrained-flow reactor, the fuel, oxygen and steam are injected at the top of the gasifier through a co-annular burner. The gasification usually takes place in a refractory-lined vessel which operates at a pressure of about 40 bars to 60 bars and a temperature in the range of from 1300° C. to 1700° C.
There are two types of gasifier wall construction: refractory and membrane. The gasifier conventionally uses refractory liners to protect the reactor vessel from corrosive slag, thermal cycling, and elevated temperatures that range from 1400° C. up to 1700° C. The refractory material is subjected to the penetration of corrosive components from the generation of the synthesis gas and slag and thus subsequent reactions in which the reactants undergo significant volume changes that result in degradation of the strength of the refractory materials. The replacement of refractory linings can cost several millions of dollars a year and several weeks of downtime for a given reactor. Up until now, the solution has been the installation of a second or parallel gasifier to provide the necessary continuous operating capability, but the undesirable consequence of this duplication is a significant increase in the capital costs associated with the unit operation.
On the other hand, membrane wall gasifier technology uses a cooling screen protected by a layer of refractory material to provide a surface on which the molten slag solidifies and flows downwardly to the quench zone at the bottom of the reactor. The advantages of the membrane wall reactor include reduced reactor dimensions as compared to other systems; improved on-stream time of 90%, as compared to on-stream time of 50% for a refractory wall reactor; elimination of the need to have a parallel reactor to maintain continuous operation as in the case of refractory wall reactors; and the build-up of a layer of solid and liquid slag that provides self-protection to the water-cooled wall sections.
In a membrane wall gasifier, the build-up of a layer of solidified mineral ash slag on the wall acts as an additional protective surface and insulator to minimize or reduce refractory degradation and heat losses through the wall. Thus the water-cooled reactor design avoids what is termed “hot wall” gasifier operation, which requires the construction of thick multiple-layers of expensive refractories which will remain subject to degradation. In the membrane wall reactor, the slag layer is renewed continuously with the deposit of solids on the relatively cool surface. Further advantages include short start-up/shut down times; lower maintenance costs than the refractory type reactor; and the capability of gasifying feedstocks with high ash content, thereby providing greater flexibility in treating a wider range of coals, petcoke, coal/petcoke blends, biomass co-feed, and liquid feedstocks.
There are two principal types of membrane wall reactor designs that are adapted to process solid feedstocks. One such reactor uses vertical tubes in an up-flow process equipped with several burners for solid fuels, e.g., petcoke. A second solid feedstock reactor uses spiral tubes and down-flow processing for all fuels. For solid fuels, a single burner having a thermal output of about 500 MWt has been developed for commercial use.
In both of these reactors, the flow of pressurized cooling water in the tubes is controlled to cool the refractory and ensure the downward flow of the molten slag. Both systems have demonstrated high utility with solid fuels, but not with liquid fuels.
For production of liquid fuels and petrochemicals, the key parameter is the ratio of hydrogen-to-carbon monoxide in the dry synthesis gas. This ratio is usually between 0.85:1 and 1.2:1, depending upon the feedstock characteristics. Thus, additional treatment of the synthesis gas is needed to increase this ratio up to 2:1 for Fischer-Tropsch applications or to convert carbon monoxide to hydrogen through the water-gas shift reaction represented by CO+H2O→CO2+H2. In some cases, part of the synthesis gas is burned together with some off gases in a combined cycle to produce electricity and steam. The overall efficiency of this process is between 44% and 48%.
While gasification processes are well developed and suitable for their intended purposes, their applications in conjunction with other refinery operations have been limited.
It is therefore an object of this invention to provide an integrated process for the disposal of solvent deasphalting bottoms recovered from a solvent deasphalting process that is economically valuable and environmentally friendly, and that is capable of producing a synthesis gas and/or hydrogen that can be used as a feedstream for other processes in the same refinery, and to generate electricity.